Its easy to blame sloppy track for derailments especially around turnouts or crossings but its often nothing to do with the track itself.
Take a more concerted look at the locos and rolling stock and check their behaviour first.
I had an issue with a tightly curved turnout and some wagons and a loco when crossing it. Lets take the turnout first and look at its construction.
Track widths across the turnout are within limits, but the 0-4-2 loco i was using jumped the track at the place shown in red. On closer inspection not when the driving wheels reached that point but when the pony truck wheels did. This made it look like the frog was at fault making the loco derail at that point. But on closer inspection it was the pony truck axle causing the problem. Not because it was set wrong but because its ability to flex properly was impeded by suspension springs and lack of side play.
The net result forced the inside wheel off the guiding rail and jammed the loco forcing the driving wheels to derail. To avoid the trapping I opened out the pony truck wheels back to back to 41.5mm and in filled the frog of the turnout with a slither of plastic. The outcome was that it now glided over the turnout with no issues.
The next part of this problem is caused by buffer locking. The best way to erase this to my mind is by using Kaydee or similar couplings set just enough to stop the buffers interlocking and derailing the stock. Here is my attempt at doing this. I have used O gauge sized couplings not gauge 1,
Long length couplings #746 will give the right clearance to solve the buffer lock problem. As far as the Tower coach is concerned the bogies needed their clearances between themselves and the chassis increasing slightly to allow for the tight curve. The 14xx loco is also a little light on the curved section of the tracks and prone to wheel slippage. This can be cured to some extent by adding lead weight to the boiler or water tank spaces.
Its not always what it seems